Fatty acid biosynthesis by a particulate preparation from germinating pea

Abstract

1. Fatty acid synthesis was studied in microsomal preparations from germinating pea (Pisum sativum). 2. The preparations synthesized a mixture of saturated fatty acids up to a chain length of C₂₄ from [¹⁴C]malonyl-CoA. 3. Whereas hexadecanoic acid was made de novo, octadecanoic acid and icosanoic acid were synthesized by elongation. 4. The products formed during [¹⁴C]malonyl-CoA incubation were analysed, and unesterified fatty acids and polar lipids were found to be major products. [¹⁴C]Palmitic acid represented a high percentage of the acyl-carrier protein esters, whereas ¹⁴C-labelled very-long-chain fatty acids were mainly present as unesterified fatty acids. CoA esters were minor products. 5. The addition of exogenous lipids to the incubation system usually resulted in stimulation of [¹⁴C]malonyl-CoA incorporation into fatty acids. The greatest stimulation was obtained with dipalmitoyl phosphatidylcholine. Both exogenous palmitic acid and dipalmitoyl phosphatidylcholine increased the amount of [¹⁴C]-stearic acid synthesized, relative to [¹⁴C]palmitic acid. Addition of stearic acid increased the amount of [¹⁴C]icosanoic acid formed. 6. [¹⁴C]Stearic acid was elongated more effectively to icosanoic acid than [¹⁴C]stearoyl-CoA, and its conversion was not decreased by addition of unlabelled stearoyl-CoA. 7. Incorporation of [¹⁴C]malonyl-CoA into fatty acids was markedly decreased by iodoacetamide and 5,5′-dithiobis-(2-nitrobenzoic acid). Palmitate elongation was sensitive to arsenite addition, and stearate elongation to the presence of Triton X-100 or fluoride. The action of fluoride was not, apparently, due to chelation. 8. The microsomal preparations differed from soluble fractions from germinating pea in (a) synthesizing very-long-chain fatty acids, (b) not utilizing exogenous palmitate–acyl-carrier protein as a substrate for palmitate elongation and (c) having fatty acid synthesis stimulated by the addition of certain complex lipids.